Prior to 1970, acetic acid was made from methanol and carbon monoxide using cobalt catalysts. Rhodium catalysts, which were developed later by Monsanto, for acetic acid synthesis, are much more active than cobalt catalyst, thus allow lower reaction pressures and temperatures. Most importantly, the rhodium catalysts give high selectivities to acetic acid.
One problem with the original Monsanto process is that a large amount of water (about 14%) is needed to produce hydrogen in the reactor via the water-gas shift reaction. Water and hydrogen help to convert Rh(III) and Rh(II) species to the active Rh(I) species. The large amount of water increases the amount of hydrogen iodide present in the reaction system, which is highly corrosive. Further, removing a large amount of water from the acetic acid product is costly.
In the late 1970s, the Monsanto process was improved by including an iodide salt such as lithium iodide to the rhodium catalyst system. Lithium iodide increases the catalyst stability by minimizing the side reactions that produce inactive Rh(III) species and therefore the amount of water needed is reduced. However, the high concentration of lithium iodide promotes stress crack corrosion of the reactor vessels. Furthermore, the use of iodide salts increases the iodide impurities in the acetic acid product.
In the late 1990s, another rhodium carbonylation catalyst system was developed. The catalyst system uses a pentavalent Group VA oxide such as triphenylphosphine oxide as a catalyst stabilizer instead of an iodide salt. The catalyst system not only reduces the amount of water needed but also increases the carbonylation rate and acetic acid yield. See U.S. Pat. No. 5,817,869.
One challenge still facing the industry is that lowering water concentration in the methanol carbonylation results in increased aldehydes formation. Methods for removing aldehydes from acetic acid products are known. For instance, U.S. Pat. No. 6,667,418 discloses a method for reacting aldehydes impurities with air, hydrogen peroxide and other free radical initiators in an integrated acetic acid production process at an elevated temperature. Introducing free radical initiators in acetic acid production processes is undesirable because some free radical initiators are explosive and may present safety concerns. U.S. Pat. No. 7,345,197 discloses a method for removing aldehyde impurities from acetic acid that comprises extracting the aldehyde impurities from a methyl iodide solution such as the decanter heavy phase with a polyol. After the aldehyde impurities are removed, the methyl iodide heavy phase is recycled to the carbonylation reaction. U.S. Pat. No. 7,485,749 discloses another method for removing aldehyde impurities from an acetic acid stream in an acetic acid preparation process. The method comprises reacting aldehyde impurities with a hydroxyl compound in a drying distillation column or a combined column to form corresponding acetals. U.S. Pat. No. 7,524,988 discloses a method for preparing acetic acid comprising reacting a portion of the heavy, organic phase comprising mostly methyl iodide and aldehyde impurities with a hydroxyl compound to convert the aldehydes into acetals. The acetals are disposed of as waste.
There is a continued need for improved methods for producing acetic acid with low level of aldehydes.